Sulfur in motor fuels such as gasoline and diesel fuel is an important pollutant. Its concentration has been regulated over the past several years so as not to exceed levels in the range of 500 parts per million (ppm). Recent government regulations worldwide will reduce the acceptable sulfur contents of gasoline and diesel fuel to below 50 ppm with specific regulatory levels set at 30 and 15 ppm, for example, to be enforced in the next two or three years. In order to ensure that the regulated concentration levels are not exceeded, petroleum products are subjected to both laboratory and on-stream analysis during their processing and production. Sulfur and nitrogen also occur at parts per million levels in beverages and likewise need to be monitored. Suitable methods for use at concentrations down to 10 ppm and below are “pyro-UV fluorescence” (ASTM D 5453) and “pyro-electrochemical” (ASTM D 6428) methods, each method incorporated herein by reference. Nitrogen often occurs in petroleum products and (incidentally) can be measured by “pyro-chemiluminescence” (ASTM D4629) or “pyro-electrochemical” (ASTM D 6366) methods, each method incorporated herein by reference. In all these methods, a small fixed volume of sample is thermally oxidized (“pyrolyzed”) and the combustion products are analyzed for SO2 or NO. The concentrations of these gases are measured by either UV fluorescence spectrometry (SO2), chemiluminescence (NO), or by electrochemical detectors specific for SO2 or NO.
According to the ASTM method directed toward pyro-electrochemical techniques, a fixed volume, usually 5-20 microliters, of liquid sample is injected into the pyrolyzer along with an inert carrier gas, usually argon at a flow rate of about 130-160 sccm (standard cubic centimeters per minute) and including some oxygen, about 10-30 sccm. The liquid vaporizes and then reaches the combustion zone where another flow of oxygen, about 450-500 sccm, the “pyrolysis-gas”, is introduced and effects complete thermal oxidation at about 1050° C. The reactor is a quartz tube heated by a tube furnace. The flow rate of liquid sample should never exceed about 4 μl/s (microliters/second), otherwise the combustion process will be starved of oxygen and soot formation (or “sooting”) will occur, that is, the internal surfaces downstream of the hot zone will be covered with soot. The ASTM methods specify a flow rate of 1 μl/s. The gas output from the pyrolyzer is a mixture of the inert carrier gas (about 20 vol %), unconsumed oxygen (about 60 vol %), carbon dioxide (CO2)(about 10 vol %), water vapor (about 10 vol %) and ppm levels of SO2. The dewpoint is 45-50° C., so the gas lines are usually heat traced and/or the water vapor content is reduced to prevent condensation. Water vapor can be reduced without affecting the SO2 content by means of a permeation dryer which operates on the principle of absorption-desorption of water vapor through a membrane. The conditioned gas mixture is then fed to the SO2 detector. A typical 20 μl sample takes some 20 seconds to inject and passes through the pyrolyzer and other gas sample plumbing in about one minute. The SO2 concentration at the detector starts at zero just before the injection, rises to a maximum and then falls off to zero. The rates of rise and fall depend on the various flow rates and gas mixing, and on any molecular exchange reactions that the SO2 undergoes at surfaces with which it comes into contact with. The detector response ideally follows this same profile. The actual detector response will be less than ideal, so additional broadening of the time profile will occur. In practice, the whole SO2 signal from a given injection will extend over 2-5 minutes. This signal is integrated and is directly proportional to the total amount of sulfur in the original sample. As long as the sample volume remains constant, the SO2 signal is proportional to sulfur content of the original sample. “Continuous” analysis is accomplished by automating the sample injection procedure.
The electrochemical detectors have the great advantages of simplicity and low cost. However, the SO2 sensor, while not sensitive to NO, is highly sensitive to any NO2 in the pyrolyzed gas stream, having an NO2 response equal to upwards of about −100% of the SO2 response. The pyrolysis occurs in a quartz tube held at about 1050° C. in a tube furnace. At this temperature gas chemistry indicates that the thermal equilibrium between the nitrogen combustion products NO and NO2 is almost completely driven toward about 100% NO, so production of NO2 is not expected and was not mentioned in any of the relevant ASTM methods.
We have surprisingly and unexpectedly found that the SO2 signal from diesel fuel, for example, containing about 50 ppm nitrogen and about 20 ppm sulfur is strongly suppressed compared to that from diesel containing about 20 ppm sulfur and about zero nitrogen. Part of the sulfur signal is reduced to zero or negative values and the sensor takes as much as one hour to recover after nitrogen in no longer present in the sample. FIG. 1 shows a typical effect. It appears that some of the NO is converted to NO2 in the cooler parts of the pyrolysis tube where the thermal equilibrium favors more NO2. Also, the electrochemical cell appears to be “poisoned” by the NO2 with long-lasting effects. It is therefore desirable to prevent NO2 from reaching the SO2 detector.